Apparatus for catalytically converting fluid

ABSTRACT

Method and means of catalytically converting fluids such as exhaust gases. The fluid is passed through a catalytically active first skeletal material zone having a plurality of flow paths in the direction of flow of predetermined cross-sectional dimensions. Immediately after withdrawing the fluid from the first skeletal material zone, the fluid is passed through an adjacent second skeletal zone having a plurality of flow paths in the direction of flow of larger cross-sectional dimensions than the flow paths of the first skeletal material zone. Immediately after withdrawing the fluid from the second zone the fluid is passed through an adjacent catalytically active third zone of skeletal material having flow paths in the direction of flow of smaller cross-sectional dimensions than the flow paths of the second zone. 1BACKGROUND OF THE INVENTION The present invention is directed to the method and means of catalytically converting a fluid. More particularly, it is directed to method and means of catalytically converting a fluid such as exhaust gases by passing such fluid through a catalytically active first skeletal material zone having a plurality of flow paths in the direction of flow, through a second similar zone of larger flow paths which may or may not be catalytically inactive, and through a third catalytically active zone of smaller flow paths than the second zone. The use of catalysts in catalytically active zones to promote chemical reactions has received extensive attention in commercial processes and in the control of air pollution. For many years considerable research has been devoted to the discovery and improvement of catalytic materials which will accelerate desired chemical conversions with the idea that for most reactions there is some material or composition of material which will catalyze and promote the reactions more efficiently and economically than known methods of catalysis. Concurrent with the increasing use of catalysts has been the development of catalytic apparatus. Numerous types of such apparatus and methods of employing catalytic material have been proposed. Thus, catalytic materials have been prepared as gels, powders, pellets, and other forms and have been arranged in solid beds, layers, spaced beds, tubes, suspensions, and other manners. Such forms and manners of arrangement have been more or less satisfactory, the particular form and arrangement depending to some extent upon the particular catalytic material being used and the operation in which it is being used, but more efficient ways are constantly being sought. In recent years, the desirability of removing or converting noxious compounds of exhaust gases from automobiles, trucks, etc., has been generally well established. The unavoidable incomplete combustion of hydrocarbon fuel via gasoline or diesel engine results in a generation of substantial quantities of unburned hydrocarbons, oxides of nitrogen, and other undesirable products, which as waste products discharge into the atmosphere through the exhaust line. With the ever-increasing concentration of automobiles, particularly in urban areas, the resulting accumulation of these undesirable products in the atmosphere may reach high proportions. These combustion products are known to react with atmospheric gases to produce smog or pollution. Such waste products include, for example, saturated and unsaturAted hydrocarbons, carbon monoxide, aromatics, partially oxygenated hydrocarbons, such as aldehydes, ketones, alcohols, and acids as well as oxides of nitrogen and sulfur. In a catalytic operation, hot gases issuing from the engine exhaust manifold are passed through a catalytic zone maintained within a converter, so as to effect a more or less complete conversion of the waste products in the exhaust to a harmeless state. One of the problems of past systems which utilize honeycomb or skeletal material as the catalyst support, is the apparent loss of activity per unit length when thicknesses of material greater than a certain length are used. In other words, under certain conditions, one inch of material may result in 60 percent conversion. Under the same conditions, three inches of such material may result in only 65 percent conversion. We have found this to occur because mass transfer limiting boundary layers form in the downstream area of the honeycomb material after entry effects have disappeared. The present invention minimizes this effect.

United States Patent [191 Hervert et a1.

APPARATUS FOR CATALYTICALLY CONVERTING FLUID Inventors: George L.Hervert, Woodstock;

Robert D. Carnahan, Barrington; Karl J. Youtsey, Chicago, all of 111.

Universal Oil Products Company, Des Plaines, 111.

Filed: Oct. 4, 1971 Appl. No.: 186,126

Assignee:

References Cited UNITED STATES PATENTS 8/1968- Talsma 423/214 X 9/1965Ruff et al. 252/477 R 7/1933 Finn 23/288 F [11 3,785,781 Jan. 15, 1974Primary ExaminerG. O. Peters Attorney-James R. Hoatson, Jr. et a1.

[57] ABSTRACT Method and means of catalytically converting fluids suchas exhaust gases. The fluid is passed through a catalytically activefirst skeletal material zone having a plurality of flow paths in thedirection of flow of predetermined cross-sectional dimensions.Immediately after withdrawing the fluid from the first skeletal materialzone, the fluid is passed through an adjacent second skeletal zonehaving a plurality of flow paths in the direction of flow of largercross-sectional dimensions than the flow paths of the first skeletalmaterial zone. Immediately after withdrawing the fluid from the secondzone the fluid is passed through an adjacent catalytically active thirdzone of skeletal material having flow paths in the direction of flow ofsmaller cross-sectional dimensions than the flow paths of the secondzone.

3 Claims, 3 Drawing Figures PATENTEUJM 15 m4 Figure S R 0 T M V W GEORGEL. HERVERT ROBERT D. CARNAHAN KARL J. YOUTSE) APPARATUS FORCATALYTHCALLY CONVERTKNG FLUKE) BACKGROUND OF THE INVENTION The presentinvention is'directed to the method and means of catalyticallyconverting a fluid. More particularly, it is directed to method andmeans of catalytically converting a fluid such as exhaust gases bypassing such fluid through a catalytically active first skeletalmaterial zone having a plurality of flow paths in the direction of flow,through a second similar zone of larger flow paths which may or may notbe catalytically inactive, and through a third catalytically active zoneof smaller flow paths than the second zone.

The use of catalysts in catalytically active zones to promote chemicalreactions has received extensive attention in commercial processes andin the control of air pollution. For many years considerable researchhas been devoted to the discovery and improvement of catalytic materialswhich will accelerate desired chemical conversions with the idea thatfor most reactions there is some material or composition of materialwhich will catalyze and promote the reactions more efficiently andeconomically than known methods of catalysis. Concurrent with theincreasing use of catalysts has been the development of catalyticapparatus. Numerous types of such apparatus and methods of employingcatalytic material have been proposed. Thus, catalytic materials havebeen prepared as gels, powders, pellets, and other forms and have beenarranged in solid beds, layers, spaced beds, tubes, suspensions, andother manners. Such forms and manners of arrangement have been more orless satisfactory, the particular form and arrangement depending to someextent upon the particular catalytic material being used and theoperation in which it is being used, but more efficient ways areconstantly being sought.

In recent years, the desirability of removing or converting noxiouscompounds of exhaust gases from automobiles, trucks, etc., has beengenerally well established. The unavoidable incomplete combustion ofhydrocarbon fuel via gasoline or diesel engine results in a generationof substantial quantities of unburned hydrocarbons, oxides of nitrogen,and other undesirable products, which as waste products discharge intothe atmosphere through the exhaust line. With the everincreasingconcentration of automobiles, particularly in urban areas, the resultingaccumulation of these undesirable products in the atmosphere may reachhigh proportions. These combustion products are known to react withatmospheric gases to produce smog or pollution. Such waste productsinclude, for example, saturated and unsaturated hydrocarbons, carbonmonoxide, aromatics, partially oxygenated hydrocarbons, such asaldehydes, ketones, alcohols, and acids as well as oxides of nitrogenand sulfur. In a catalytic operation, hot gases issuing from the engineexhaust manifold are passed through acatalytic zone maintained within aconverter, so as to effect a more or less complete conversion of thewasteproducts in the exhaust to a harmeless state.

One of the problems of past systems which utilize honeycomb or skeletalmaterial as the catalyst support, is the apparent loss of activity perunit length when thicknesses of material greater than a certain lengthare used. in other words, under certain conditions, one inch of materialmay result in 60 percent conversion.

Under the same conditions, three inches of such mate rial may result inonly percent conversion. We have found this to occur because masstransfer limiting boundary layers form in the downstream area of thehoneycomb material after entry effects have disappeared. The presentinvention minimizes this effect.

SUMMARY OF THE INVENTION cally active first skeletal structure having aplurality of flow paths in the direction of flow of predeterminedcross-sectional dimension; an adjacent second skeletal structurecontacting said first skeletal structure and having a plurality of flowpaths in the direction of fluid flow of larger cross-sectionaldimensions than the flow paths of said first skeletal structure,and anadjacent catalytically active third skeletal structure contacting saidsecond skeletal structure having a plurality of flow paths in thedirection offlow of smaller cross-sec tional dimensions than the flowpaths of said second skeletal structure.

The second skeletal structure may be catalytically inactive, preferably,however, the skeletal strucutre is made catalytically active to furtherincrease the capability of the element.

Another aspect of this invention provides for a catalytic converter forconverting fluids comprising in combination: (a) an outer housing; (b)inlet means connected to said housing for introducing fluid therein; (c)outlet means connected to said housing for discharging of convertedfluid therefrom; and, (d) a catalyst elemerit supported in said housing;(c) said catalyst ele. ment including a catalytically active firstskeletal structure having a plurality of flow paths in the direction offlow of predetermined cross-sectional dimensions; an

7 adjacent second skeletal structure contacting said first skeletalstructure having a plurality of flow paths in the direction of flow oflarger cross-sectional dimensions than the flow paths of said firstskeletal structure; and an adjacent catalytically active third skeletalstructure contacting said second skeletal structure having a pluralityof flow paths in the direction of flow of smaller cross-sectionaldimensions than the flow paths of said second skeletal structure.

in a preferred embodiment, barrier means is pro- .vided upstream of theinlet perforate side of the catalyst element for preventing a fluid flowthrough the perimeter openings of the structure. Thus, in effe ct, thereis an insulating portion around the periphery of the skeletal structureto prevent loss of heat through the sides" of the hdusingQAgain, thesecond skeletal structure may be made to be catalytically active.

In another aspect of the present invention provides for a method ofcatalytically converting fluids comprising the steps of: (a) passingsaid fluid through a first catalytically active zone of skeletalmaterial having flow paths in the direction of flow of predeterminedcross-sectional dimensions; (b) immediately upon withdrawing the fluidfrom the first catalytically active zone passing said fluid through anadjacent second zone of skeletal material having flow paths in thedirection of flow of larger predetermined cross-sectional dimensionsthan the flow paths of said first catalytically active zone; and (c)immediately after withdrawing the fluid from the said adjacent secondzone of skeletal material, passing said fluid through an adjacentcatalytically active third zone of skeletal material having flow pathsin the direction of flow of smaller cross-sectional dimensions than theflow paths of the immediately adjacent zone of skeletal material.

Reference to the accompanying drawing and the following descriptionthereof will serve to more fully illustrate the present invention aswell as to set forth additional advantageous features in connectiontherewith.

DESCRIPTION OF THE DRAWING FIG. 1 of the drawing is an elevationalsectional view of a preferred embodiment of the converter of the presentinvention.

FIG. 2 of the drawing is a pictorial view of honeycomb structure whichmay be utilized in connection with the present invention.

FIG. 3 of the drawing is a pictorial view of one embodiment of thecatalytic element of the present inventon.

Referring now to FIG. I of the drawing there is shown an elevationalview of a catalytic reactor or converter which may be used for theconversion of exhaust gases. Converter I is comprised of an outerhousing 2 havkng a central tubular portion 3 and two end closuresections 4 and 5. Because of ease in fabrication, the preferredcross-section of the housing 2 is circular; however, this should not belimiting upon the present invention. Other shapes are contemplated suchas rectangular, oval and the like. Reactor I is further comprised of aninlet 6 and an outlet 7 for introducing and discharging the gasestherein. Of course both inlet and outlet means communicate through thehousing 2 into the interior thereof. End closure means 4 and 5 haveinwardly facing flanged end sections 8 and 9 respectively which are usedto support a catalyst element 30. Openings l and II are provided in endclosure means 4 and respectively.

The catalyst element 30 is comprised of a catalytically active firstskeletal structure 12 having a plurality of flow paths 31 extendingtherethrough, a second skeletal strucure 13 having a plurality of'flowpaths 32 extending therethrough, a catalytically active third skeletalstructure 14 having a plurality of flow paths 33 extending therethrough,a fourth skeletal structure 15 having a plurality of flow paths 34extending therethrough and a catalytically active fifth skeletalstructure 16 having a plurality of flow paths 35 extending therethrough.Of course all of the flow paths are arranged to provide communicationbetween adjacent skeletal structures in the general direction of thefluid flow. It should be noted from the schematic illustration that thechannels or flow paths in elements 12, i4, and 16 are of smallercross-sectional dimensions than the flow paths of elements 13 and 15.The schematic illustration shows an exaggeration of what the size ofthese path openings would be. In typical honeycomb material, forexample, the smaller sized paths may be disposed with perhaps about 250to about 500 channels or flow paths per square inch while the largersize flow paths may be disposed at a rate of about to about I50 flowpaths per square inch.

The skeletal structure, which is sometimes referred to as a honeycombmaterial, is well known to those skilled in the art. There are variouskinds on the market and major US. manufacturers include American LavaCorporation, a subsidiary of 3M Corporation, E. I. Du- Pont Inc., andCorning Glass Company. Honeycomb or skeletal material as used herein ingeneral refers to a unitary inert refractory skeletal structure which ischaracterized by having a large plurality of gas flow paths extendingthrough the material in the direction of fluid flow. The openings may besubstantially parallel and extend through the support from one side tothe opposite, with such openings being separated from one another bypreferably thin walls defining openings. Reference should be made toFIG. 2 of the drawing for one example of such a structure where it isseen that the honeycomb is comprised of a layer of corrugated film 20and a sheet adhere together to form the honeycomb material. The resultis that a plurality of unobstructed flow paths 23 are formed. Referenceshould be made to Johnson US. Pat. No. 3,344,925 for a more completedescription of the method of making this particular hoenycombconfiguration. Alternatively, a network of flow paths may permeate thestructure so as to form a tortuous flow path such as a cross flow gradeof honeycomb materials sold under the registered trademark of Torvexmanufactured by DuPont Corp. The particular honeycomb structure shown inFIG. 2 is an American Lava configuration.

Typically, the path openings of honeycomb material are distributedacross the entire face of the material and are subject to initialcontact with the gas or fluid to be reacted. The paths can be of anyshape and size consistent with the desired superficial surface area. Thecross-sectional shape of the path can be, for example a trapezoid,rectangle, square, sine-wave, circle, and other cross-section that showsrepeating patterns that can be described as a honeycomb, corrugated, orlattice material, since it is not the intent of this invention to limitthe shape and size.

Likewise, it is not intended to limit the honeycomb material to anyparticular composition. The main features of the material to be used isthat it should be able to withstand the stresses it will experience inthe reactor to which it is used. Also, preferably, it should be able tosupport a catalyst or act as a catalyst. Suitable materials includesilica-alumina-magnesia (cordierite), aluminum oxide, lithium, aluminumsilicates, magnesium alumina silicates and the like.

As was mentioned before, I preferably honeycomb structures I2 and 14 aremade catalytically active to increase the overall reactor efficiency.The honeycomb structures generally must be treated to obtain catalyticactivity. The catalyst used for the reaction will have to be one tocatalyze that particular reaction. It may be any one of the well knowncatalyst materials used; as for example, suitable oxidation catalystsincluding the oxides of the meals of Groups I, V VII and VIII of thePeriodic Table, particularly chromium, copper, nickel and platinum. Theapplication of this catalyst to the honeycomb support can be effected inmany ways, for

example by immersing the structure in an aqueous solution of awater-soluble inorganic salt or salts of the particular metal or metals,with agitation. The metal oxide can be reduced, if the metal formcatalyst is desired, by

We have found that when hoheycomb is used as a catalyst or catalystsupport, most of the conversion takes place in tee entry regionof thematerial. This is because typically entry flow into a catalyst zone suchas a skeletal structure as illustrated in H6. 2 results in an initialsquare velocity profile normal to the main flow direction. The squareprofile is converted to a parabolic one after a characteristic entrylength for zones in which laminar flow is at normal fluid condition. Theestablishment of a parabolic velocity profile results in a boundarylayer consisting of a stopped or slowly moving fluid in the immediateneighborhood of the catalyst surface. The extent of reaction in thisstagnant layer leads to the rapid establishment of a steep concentrationgradient with respect to the core of the fluid flow. The characteristicentry length is greater for entrant turbulent flow than for entrantlaminar flow. An increased overall reaction or conversion rate isachieved when a square reactant concentration profile is maintained inthe reactor, regardless of whether the dynamic condition of the flow isturbulent of laminar, although turbulent flow will insure such asprofile for a greater distance through the channels. Thus, the optimumdesign of the first catalytically active zone limits the zone length ofthe characteristic entry length. The length of this active elementshould be such that a mass transfer limiting concentration gradient isjust established and the length of the element i3 is such that a squareconcentration profile is reestabished prior to entry into the nextcatalytically active zone M. The larger cros-section of dimensions ofthe path openings 32 in zone 13 interrupts the boundary layer of zone 12and allows the square concentration profile to be reestablished prior toentry into zone M of smaller crosssectional dimensions. The squareconcentration profile is reestablished by either or both of thefollowing mechanisms: mixing by diffusional mass transfer andmixingcaused by element interfacing mismatching. This same analysis is madewith respect to element M, 15 and 116.

That is, in element M the square reactant concentration profilediminishes and just as a mass transfer limiting concentration gradientis established the fluid is introduced into element 15 wherein thesquare concentration profile is reestablished prior to entry intoelement H6.

The advantages of this particular arrangement over a spaced honeycombconcept is that thinner active elements can be used without loss ofreactor mechanical strength because element 30 may have greatermechanical strength than any of the individual elements have supportedby themselves. Also the channel diameters in the respective elements canbe varied to acieve speciflc design objectives. For example, the initialactive element might consist of a material with very smallchanneldiameters to promote reactor light offj Also the assembly infabrication procedure is greatly simplifie d.

The honeycomb material may be placed individually into the reactor or onthe other hand may be preformed into a single element prior to insertioninto the reactor. For example in H6. 3 there is shown a sequence ofelements that are arranged inthe desired order. The correspondingchannel sizes are not shown but it is assumed tht they fall within thescope of the invention.

That is element 5% has smaller channel sizes than element 5H and element52 has smaller channel sizes than 51. The elements 52 may then bewrapped with a ceramic wrapping material 55 which may be in the form ofthe same material used to make the elements themselves. ifnecessary thewrapping material may be heated to set the shape thereof. The elements5t), 51 and 52 may be impregnated with the catalytically active materialprior to the wrapping of the wrapper 55 but on the other'hand a packagemay be impregnated as one piece and heat treated thereafter.

Referring again to FIG. 1 of the drawing it is noted that the honeycombmaterial is spaced from the housing and in the space there is providedpacking 17 to take up any relative expansion of the housing and thehoneycomb material and also to provide for greater dimensional tolerancein manufacturing procedures. It is also noted that the flanged portion 8of the end closure means 4i overlaps the peripheral edge of thehoneycomb material. This has the effect of blocking flow material usedis such that the square reactant profile goes parabolic aftr a shortdistance into the material. Thus, by using optimum lengths of honeycombmaterial it was seen that the square reactant concentration profile canbe maintained by reestablishment through much of the length of element30.

After the fluid leaves the first hoenycomb material it is subjected to aredistribution and mixing to reestablish a substantially square reactantconcentration profile prior to entry into the third element. This isrepeated through succeeding elements of material.

From the foregoing description it is seen that the present inventionprovides for the method and means of catalytically converting a fluidwhich comprises the steps of passing such fluid through a catalyticallyactive first skeletal zone having a plurality of flow paths in thedirection of flow, then through a second similar flow of larger flowpaths, and finally through a third zone of smaller flow paths than inthe second zone. The

.embodiment of FIG. 1 illustrates an expanded version of the broaderconcept with two additional zones added. it is contemplated that thesteps be repeated more than shown and stilll fall within the scope ofthe present invention. It should be understood that many .variations ofstrucure configurations illustrated in the drawing are possible withoutdeparting from the essential features of this invention. The presentinvention is not intended to be limited to any particular description orexemplary arrangement disclosed merely to describe the invention morefully. We claim as our invention:

d. a catalyst element supported in said housing; and,

e. said catalyst element including a catalytically active first skeletalstructure having a first plurality of flow paths in the direction offlow of predetermined cross sectional dimensions; said first pluralityof flow paths comprising from about 250 to skeletal structure) and anadjacent catalytically active third skeletal structure contacting saidsecond skeletal structure having a plurality of flow paths in thedirection of flow of cross-sectional dimensions the same as said firstdefined plurality of flow paths.

2. The converter of claim 1 further characterized in that said secondskeletal structure is catalytically active.

3. The converter of claim 1 further characterized in that barrier meansis provided around the peripheral edge of the first skeletal structurefor blocking fluid flow through the peripheral flow paths thereof.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,785,781 Dated January 15, 197R.

I ve tm-( George L. Hervert et a1.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 7 line 1h, cancel "(than the flow paths "of said first" Column 8line- 1, cancel "skeletal structure)" Signed and sealed this 18th day ofJune l9-7L EDWARD M.FLETCHER ,JR.

Commissioner of Patents Attesting Officer FORM Po-wso (10-69) USCWWDC16.

I I Q U. 5. GOVERNMENT PRINTING OFIIC! l9. 0-566-SSL

2. The converter of claim 1 further characterized in that said secondskeletal structure is catalytically active.
 3. The converter of claim 1further characterized in that barrier means is provided around theperipheral edge of the first skeletal structure for blocking fluid flowthrough the peripheral flow paths thereof.